Maybe it has changed since the last time I did that flight was a year or so ago. If you have the SIN boarding pass...you will be asked to remain on board, if you have two boarding passes HKG-BKK and BKK-SIN, you may need to coordinate with the ground staff. Although for me the ISM asked me to stay on board and called a ground staff to deal with my BKK-SIN boarding pass.

However on the HKG-SIN-BKK, all passengers are asked to deplane.